It is often necessary to measure the average surface roughness of a material. There are various definitions of surface roughness and a corresponding number of quantitative parameters. Essentially however, the measurement indicates height variations on the surface of the material under inspection and the average of these height variations is indicated by the parameter Ra.
There are a number of methods used for measuring surface roughness of a material. Some involve the use of a stylus that is drawn over the surface of the material with the vertical deflection of the stylus providing a measure of the surface roughness. However with this method, the resolution of the measurement is limited to the dimensions of the stylus and the contact pressure of the stylus on the surface of the material can damage the surface. Other methods involve illuminating the surface with radiation and then analysing the contrast in the resulting speckle that is generated. However, each of these methods only provides a limited measurement range and has a limited resolution.
An alternative method comprises illuminating the surface of a material using a collimated beam of radiation from two slightly different incident angles. The radiation incident upon the surface at the two different incident angles is then correlated to determine the surface roughness and this correlation method is found to provide an extended measurement range compared to the alternatives.
In order to illuminate the material surface at slightly differing incident angles, either the material is rotated or the beam of radiation is deflected. The former is not always practical for measurement of large pieces arranged in-situ. For the latter, and referring to FIG. 1 of the drawings, the principal approach is to reflect a portion of the beam 10 using a beam-cube beam splitter 11, and to re-direct the reflected beam onto the test piece 12 using a mirror 13. Experimental investigations have shown that the closest agreement between the measured values of surface roughness using the various techniques occurs when the difference in incident angles is of the order of ˜1°. However, the physical size of the beam splitter 11 and mirror 13 presents a lower limit on the difference in the incident angles (arising from the lower limit on the physical path length of the reflected beam from the original) of the beams used to illuminate the surface, particularly in a limited space, and thus presents a lower limit to the error which can be achieved when making the measurement of surface roughness.